1. Field of the Invention
The present invention relates to an image display device and more particularly to a liquid crystal display (LCD) and its driving method in which an image signal to be fed to a liquid crystal panel is produced using a reference gray-scale voltage and gray-scale data.
The present application claims priority of Japanese Patent Application No. 2001-136740 filed on May 7, 2001, which is hereby incorporated by reference.
2. Description of the Related Art
In an LCD, display of an image is performed by using a liquid crystal panel as a display device. The liquid crystal panel is so configured that a first glass substrate on which a pixel electrode made up of a transparent electrode is placed in a manner to correspond to pixels arranged in a matrix form on a display surface faces a second glass substrate on which a common electrode made up of a transparent electrode with a liquid crystal substance being a crystalline liquid that provides optical anisotropy produced by an electric field put between the first and second glass substrates in a hermetically sealed manner, and polarizers whose polarizing planes intersect each other at right angles are mounted on both the glass substrates. Light and shade are displayed for every pixel by driving the pixel electrode from a row direction of and from a column direction on a panel screen, thereby changing a degree of optical anisotropy of the liquid crystal substance on the pixel electrode and changing transmittance of light which further changes luminance of transmissive-light emitted from a backlight being mounted on a rear surface. Moreover, color display is performed by arranging the pixel electrode of each pixel for each of three primary colors made up of red (R), green (G), and blue (B) color and by mounting, on the second glass substrate, a color filter for each of the pixel electrodes arranged for each of the R, G, and B colors and by driving the pixel electrode in row and column directions so that electric power being different for every color is applied thereto.
In this case, an image signal output from an image writing device such as a personal computer is made up of gray-scale data in which a level of brightness of an image is displayed on a logarithmic axis at equal intervals, for example, 64 shades of gray scale are represented by 6 bits of a digital signal. In an LCD, display of an image is performed by applying a voltage that changes according to gray-scale data to a liquid crystal panel, however, since a gamma (γ) characteristic value exhibiting a relation between a change in applied voltage and a change in luminance is ordinarily set to be about 2.2, the LCD has to be so configured that processing (γ correction) can be performed in a manner that a voltage to be applied corresponding to a gamma (γ) characteristic is produced from the gray-scale data. Moreover, in a normally white-type liquid crystal panel, since its transmittance is highest in a state where an applied voltage is not applied and the higher the applied voltage becomes the smaller the transmittance becomes, setting is made so that the applied voltage becomes smaller as the gray-scale date increases.
Next, configurations and operations of a conventional LCD will be described below. FIG. 10 is a schematic block diagram showing a first example of the conventional LCD. FIG. 11 is a schematic block diagram showing an example of configurations of a reference gray-scale voltage producing circuit and a signal line driving circuit employed in the conventional LCD. FIG. 12 is a diagram illustrating a gray-scale data input for the conventional LCD. FIG. 13 is a diagram showing an example of gamma characteristics in a liquid crystal panel of the conventional LCD.
The conventional LCD 11 shown as the first conventional example in FIG. 10, chiefly includes a liquid crystal panel 12, a display control circuit 13, a reference gray-scale voltage producing circuit 14, a scanning line driving circuit 15, and a signal line driving circuit 16. The liquid crystal panel 12, as described above, is so configured that wirings serving as a plurality of scanning lines 121 are mounted in a horizontal direction relative to a display surface and wirings serving as a plurality of signal lines 122 are mounted in a vertical direction relative to the display surface, wherein a pixel electrode 123 is formed at each point of intersection of each of the scanning lines 121 and each of the signal lines 122 and a TFT (Thin Film Transistor) 124 is connected between each of the pixel electrodes 123 and each of the signal lines 122 corresponding to each of the pixel electrodes 123 and a gate of each of the TFTs 124 is connected to each of the scanning lines 121. In this case, as shown in FIG. 10, one screen is so constructed that a pixel electrode 123 for a red (R) color, a pixel electrode 123 for a green (G) color, and a pixel electrode 123 for a blue (B) color each being connected through the TFT 124 to the scanning line 121 and to the signal line 122 and each being arranged in order in a horizontal direction in which a specified number of sets each being made up of the above three pixel electrodes 123 are arranged and these three pixel electrodes 123 make up one color pixel, while specified pieces of the pixel electrodes 123 for a same color each being connected through the TFT 124 to the signal line 122 and to the scanning line 121 are arranged in a vertical direction.
The display control circuit 13 transmits gray-scale data having been received from an image writing device 100 and being made up of data for a gray-scale for the R, G, and B colors so as to correspond to an arrangement of the pixel electrodes 123 in the liquid crystal panel 12 and a signal line control signal, in accordance with synchronizing data also having received from the image writing device 100 and in every scanning period, to the signal line driving circuit 16 and also transmits a scanning line control signal, in accordance with the synchronizing data, to the scanning line driving circuit 15.
The reference gray-scale voltage producing circuit 14 produces a reference gray-scale voltage required when the signal line driving circuit 16 outputs a signal having a voltage corresponding to gray-scale data to each of the signal lines 122. The scanning line driving circuit 15 outputs a scanning signal to each of the scanning lines 121 for every one field in response to a scanning line control signal. The signal line driving circuit 16 produces, in every scanning period and in response to a signal line control signal, a signal having undergone a gamma (γ) correction based on a voltage-transmittance characteristic, according to gray-scale data which has been fed from the display control circuit 13 and has been sorted and according to a reference gray-scale voltage fed from the reference gray-scale voltage producing circuit 14 and outputs the signal to each of the signal lines 122.
Moreover, each of the reference gray-scale voltage producing circuit 14 and the signal line driving circuit 16 has configurations shown in FIG. 11. FIG. 11 shows an example in which voltages corresponding to the gray-scale data are output to 1920 pieces of the pixel electrodes 123 corresponding to 640 pieces of color pixels arranged in a horizontal direction in a liquid crystal panel 12. The reference gray-scale voltage producing circuit 14 outputs a voltage obtained by dividing the reference voltage VREF using a voltage dividing circuit made up of resistors R1, R2, R3, . . . , R9, R10, and R11 through voltage followers B1, B2, . . . , B9, and B10 to the signal line driving circuit 16 as reference gray-scale voltages V0, V1, . . . , V8, and V9. In the signal line driving circuit 16, an MPX (multiplexer) 161, based on a polarity reversing pulses POL used to drive the liquid crystal panel 12 with alternating current, divides reference gray-scale voltages V0 to V9 into a set of reference gray-scale voltages V0 to V4 and a set of reference gray-scale voltages V5 to V9 and then outputs the divided voltages to a DAC (digital-analog converter) 162.
Moreover, for example, 6 bits of R-color gray-scale data DR, 6 bits of G-color gray-scale data DG, and 6-bits of B-color gray-scale data DB all being fed from the display control circuit 13 are held, in parallel, in a data register section 164 being controlled by an output, which is controlled by a horizontal start pulse HSP and a clock signal HCK, fed at each stage in a shift register section 163. The above gray-scale data DR, DG, and DB being held in parallel in the data register section 164 are transferred collectively to a latch section 165 by a latch signal STB and then are latched therein. Furthermore, gray-scale data output from the latch section 165 are level-shifted through a level shift section 166 and are transferred to the DAC 162.
The gray-scale data having been transferred to the DAC 162 undergoes the gamma correction, based on the set of the reference gray-scale voltages V0 to V4 and the set of the reference gray-scale voltages V5 to V9 all being fed from the MPX 161 and then produces a D-A (digital to analog) converted signal voltage and are output through the voltage followers F1, F2, . . . , F1919, and F1920 to each of corresponding signal lines 122.
Next, operations of the LCD 11 of the first conventional example will be described by referring to FIG. 10 to FIG. 12. FIG. 12 shows a state of input of gray-scale data fed to the LCD 11 from the image writing device 100 such as a personal computer. In this example, the liquid crystal panel 12 has 640 pieces of color pixels in a horizontal direction. Also, it shows a state in which signals made up of gray-scale data containing each set of the R, G, and B colors which is parenthesized in every scanning period being repeated 640 times are input 480 times corresponding to positions of 480 pieces of the scanning lines 121 arranged in a vertical direction in the liquid crystal panel 12. The gray-scale data for each color corresponds to a number of gray-scales in an image to be displayed, for example, 64 shades of gray is expressed by 6 bits of a digital signal. Moreover, the image writing device 100 outputs a vertical sync signal as synchronizing data in a manner that it corresponds to a display period in each field and a horizontal sync signal as the synchronizing data in a manner that it corresponds to a scanning period in each line.
The display control circuit 13 outputs gray-scale data which have been input from the image writing device 100 to the signal line driving circuit 16 according to synchronizing data in every scanning period and by data for ore scanning line 121 and a scanning line control signal to the scanning line driving circuit 15 according to the synchronizing data and a signal line control signal to the signal line driving circuit 16.
This causes the scanning line driving circuit 15 to sequentially output, according to a scanning line control signal, a scanning signal which forms one field of a screen to each of the scanning lines 121 for every vertical sync signal and therefore the TFT 124 being connected to each of the scanning lines 121 is turned ON, thus allowing a signal voltage to be applied from each of the signal lines 122 to each of the pixel electrodes 123 being connected to the scanning line 121.
Moreover, the signal line driving circuit 16 makes a gamma correction to the gray-scale data for each of the R, G, and B colors by using a reference gray-scale voltage fed from the reference gray-scale voltage producing circuit 14 so that a V-T (voltage-transmittance) characteristic value in the liquid crystal panel 12 becomes a specified gamma value and outputs a voltage corresponding to a gamma-corrected V-T characteristic value to each of the signal lines 122.
Thus, in the conventional LCD shown in FIG. 10, a signal voltage is produced presuming that a voltage used for making the gamma correction to gray-scale data for each of the R, G, and B colors is same and a V-T characteristic for each of the R, G, and B colors in the liquid crystal panel 12 is also same. However, in an actual operation of the conventional LCD 11, the V-T characteristic is different in each of the R, G, and B colors, based on luminance of a backlight, transmittance of a color filter, a difference in a characteristic of a liquid crystal or a like and therefore a gamma characteristic of an image to be displayed is made different in each of the R, G, and B colors, which causes a change in gradation in color and, as a result, a decrease in an image quality. FIG. 13 illustrates a change in the gamma characteristic for each of colors to be displayed, in the case of 64 gray-scale display, showing that the transmittance for a same gray-scale value is small (that is, the gamma value is large) in order of the G color, B color and R color.
To solve these problems, in the conventional LCD 11, a method in which data is processed in advance on a side of the image writing device and gray-scale data to which a correction has been made to compensate for such differences in the gamma characteristic as described above is output, a method in which a circuit is mounted on an input side of the LCD, which makes a gamma correction to input data by each of the R, G, and B colors, or a like are employed.
Next, another LCD as a second conventional example in which a gamma correction is made to gray-scale data on an input side is described below. FIG. 14 is a schematic block diagram showing configurations of another LCD 11A as a second conventional example. FIG. 15 is a diagram illustrating an increase in a number of gray-scales based on a gamma correction in the LCD 11A of the second conventional example.
The LCD 11A of the second conventional example, as shown in FIG. 14, chiefly includes a liquid crystal panel 12, a display control circuit 13, a reference gray-scale voltage producing circuit 14, a scanning line driving circuit 15, a signal line driving circuit 16, and an image processing circuit 17. Configurations and functions of the liquid crystal panel 12, display control circuit 13, reference gray-scale voltage producing circuit 14, scanning line control signal 15, and signal line driving circuit 16 are same as those in the first conventional example shown in FIG. 10.
The image processing circuit 17 is made up of a chip having a look-up table (LUT) for an R color signal (not shown), a look-up table (LUT) for a G color signal (not shown), and a look-up table (LUT) for a B color signal (not shown) and, by reading gray-scale data, which corresponds to each of input gray-scale data for each of the R, G, and B colors, contained in each of the look-up tables for the R, G and B color signals, performs a gamma correction to each of the R, G, and B colors and then outputs gray-scale data obtained after the gamma correction to the display control circuit 13.
Next, operations of the LCD 11A of the second conventional example will be explained by referring to FIGS. 14 and 15. The gray-scale data output from an image writing device 100 made up of a personal computer or a like is arranged in a manner as shown in FIG. 12 and, for example, 64 shades of gray are expressed by 6 bits of digitalized image signal for each of the R, G, and B colors. The image processing circuit 17 inputs gray-scale data input for each of the R, G, and B colors to the each of the LUTs for the R, G, B colors and reads gray-scale data corresponding to each of the R, G, and B colors from the LUTs for each of the R, G, B colors to display gray-scale data obtained after the gamma correction and to output them to the display control circuit 13.
The display control circuit 13, as in the case of the first conventional example, outputs gray-scale data obtained after the gamma correction in every scanning period in a manner that the gamma-corrected gray-scale data corresponds to a position of each of the scanning lines 121, to the signal line driving circuit 16 and, at the same time, outputs a scanning line control signal to the scanning line driving circuit 15 and a signal line control signal to the signal line driving circuit 16. The reference gray-scale voltage producing circuit 14, as in the case of the first conventional example, outputs a reference gray-scale voltage so that a V-T characteristic value in the liquid crystal panel 12 becomes a specified gamma value. At this point, as explained in the above first conventional example, the reference gray-scale voltage is same in each of the R, G, and B colors.
The signal line driving circuit 16 generates an output voltage corresponding to input gray-scale data obtained after the gamma correction to be produced by a DAC mounted in the signal line driving circuit 16 using a reference gray-scale voltage fed from the reference gray-scale voltage producing circuit 14 and outputs it to each of the signal lines 122.
As described above, in the conventional LCD 11A shown in FIG. 14, by performing data processing on gray-scale data being an original image signal, a gamma correction is made in every color. However, if the gamma correction is made, by data processing, to input gray-data, a number of gray levels in the gray-scale data obtained after the gamma correction becomes small. This is because the input gray-scale data is so constructed that, for example, in the case of 64 shades of gray, 64 pieces of gray-scale values correspond to 6 bits of digital data in a one-to-one relationship, however, if the corresponding relationship is changed between input data and output data by data processing in the 6 bits of digital data, a digital value being skipped in reading occurs in the output data and, as a result, gray-scale data corresponding to the digital value having been skipped in reading is not output.
Thus, in the case of the gamma correction by data processing, only gray-scale data that provides direct correspondence between input data and output data is taken out and is used and therefore all the gray-scale values contained in the gray-scale data on a side of input cannot be fully used, which causes lower quality of an image caused by a decrease in a number of gray levels in an output image.
FIG. 15 is a diagram illustrating an decrease in a number of gray levels based on a gamma correction in the LCD of the second conventional example and data conversion to gray-scale data made up of, for example, 64 gray levels is performed by following equation:Dout=INT{64×(Din/64)^(1/γd)}  (1)where “Din” denotes input gray-scale data, “Dout” denotes output gray-scale data, “γd” denotes a gamma-corrected value by data processing, “INT” denotes a symbol to make values be an integer, and “^” denotes a power. In FIG. 15, the number of gray levels that can be displayed at each of the “γd” values is shown. If γd=1, since input gray-scale data matches the output gray-scale data, no change in the number of gray levels occurs. However, if γd<1 or γd>1, the number of gray levels in an output image decreases.
In each of the conventional LCDs 11 and 11A, same reference gray-scale voltage which is produced by the reference gray-scale voltage producing circuit 14 is used for each of the R, G, and B colors. The correction corresponding to a difference in a gamma characteristic for each of the R, G, and B colors in the liquid crystal panel 12 is performed by data processing to input gray-scale data.
However, in the method in which correction of a gamma characteristic is made by data processing to gray-scale data, as described above, only a portion in which input gray-scale data directly corresponds to output gray-scale data is taken out for use, all the gray-scale data contained in an image signal cannot be used, which causes a decrease in the number of gray levels in an output image after processing of the gamma correction and lowering in an image quality to be displayed.